You said it was only running at 54c, so my first suggestion would be to download HWMonitor and have it run in the background for a whole session and see if it ever spikes beyond that. It’ll keep a log of your min/max temps and should be able to pinpoint for sure whether its a thermal issue or not. OR it might even be easier to just download Aida64 and run a stress test to see if it ever goes close to or beyond 90c. If that’s the issue, search on Youtube ‘how to apply thermal paste’ and it’ll be a ~$10 and ~15min fix.
If that isn’t it and as you said your drivers are up to date, I can only give you a few random troubleshooting suggestions that may or may not work but worth trying when at a loss:
An obvious one should be reinstalling DOTA and see if that fixes it. A similar and often effective strategy would be to reinstall the current drivers for your GPU despite being up to date, something could have gotten messed up.
You can try running it in compatibility mode by going in your files to steam>steamlibrary>steamapps>common>DOTA, left-clicking the .exe/game, select properties and go to the compatibility page. I’m a little doubtful that would be your issue but when troubleshooting I wouldn’t skip possibilities.